Specify "Unrestricted" for calculation with odd number of electrons

Questions
#1

Dear all,

I have, I think, a very simple question. It’s basically in the title.
I just want to do a few opt, freq and tddft calculations on complexes in different oxidation states and wanted to know if I have to specifically set “unrestricted = 1” for the forms with an odd number of electrons such as radical cations (1 2 in the $molecule section).

Thank you and sorry if this has been answered before.
Nils

#2

Short answer:
for DFT and TDDFT using Unrestricted=TRUE is recommended on the basis of the theoretical arguments by Pople and others. However, you do need to monitor spin-contamination of the resulting states.

The issue of spin-contamination in open-shell calculations is discussed in this tutorial chapter, which you may find helpful: http://iopenshell.usc.edu/pubs/pdf/revcc-30.4-151.pdf

#3

Thank you very much for the swift answer and the extra tutorial material!

#4

For any multiplicity other than singlet, the SCF calculation will default to unrestricted any you don’t have to specify anything.